Fluid bed vinyl acetate catalyst

ABSTRACT

A catalytically active material useful to prepare vinyl acetate monomer from ethylene, acetic acid, and an oxygen-containing gas under fluid bed conditions comprises a porous microspheroidal support containing catalytically active palladium crystallites finely dispersed within the support. This catalyst material does not require incorporation of gold to maintain activity and selectivity. A process to produce a vinyl acetate fluid bed catalyst in which catalytically active small palladium crystallites are finely dispersed within the support comprises dispersing selected metal species within the support which have an affinity to palladium to form very fine crystallites of palladium. The affinity metal species may be dispersed by impregnation onto a preformed microspheroidal support or may be intimately incorporated within the support before impregnation with a soluble palladium species.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 09/207,851, filed Dec. 8, 1998, incorporated by referenceherein.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a catalyst and catalyst support usefulin producing vinyl acetate monomer (VAM) in a fluid bed process and moreparticularly relates to an active and selective fluid-bed VAM catalystsuitable for use in a fluid bed reactor in which catalytically activemetal is well dispersed within a support.

[0003] Conventionally, vinyl acetate monomer is produced in the gasphase by reacting ethylene, acetic acid, and oxygen in the presence of asupported catalyst in a fixed bed reactor. In this type of reactor, asupport material such as silica or alumina is impregnated with acatalytic metal such as palladium in combination with gold and an alkalimetal salt, typically in the form of an acetate. A requirement of afixed bed reactor process is that the supported catalyst formed intorelatively large structural shapes such as balls and may be 2 to 50 mmin diameter or more.

[0004] In early examples of fixed-bed catalysts, palladium and gold aredistributed more or less uniformly throughout the carrier, e.g., U.S.Pat. Nos. 3,275,680, 3,743,607 and 3,950,400 and Great Britain PatentNo. 1,333,449 and South African Patent No. 687,990. Since gaseousreactants do not diffuse significantly into the large fixed-bed catalyststructure, much of the expensive catalytic metal components in theinterior of the catalyst were not useful. Subsequently, fixed-bedcatalysts were developed in which most of the catalyst metals weredeposited onto the outer shell of the supported catalyst. For example,Great Britain Patent No. 1,500,167 describes a catalyst structure inwhich at least ninety percent of the palladium and gold is distributedin that part of the carrier particle which is not more than thirtypercent of the particle radius from the surface. In addition, GreatBritain Patent No. 1,283,737 teaches that the degree of penetration intothe porous carrier can be controlled by pretreating the porous carrierwith an alkaline solution of, for example, sodium carbonate or sodiumhydroxide. Another approach to produce an active catalyst is describedin U.S. Pat. No. 4,048,096 and other methods of producingshell-impregnated catalyst are disclosed in U.S. Pat. Nos. 4,087,622 and5,185,308. Shell impregnated catalysts containing elements in additionto palladium and gold such as lanthanide compounds include U.S. Pat. No.5,859,287 and WO 99/29418. In other fixed bed catalysts described inEP-A-0723810, a silica support may be impregnated with a Group IA, IIA,IIIA, or IVB metal salt and then calcined before addition of palladiumand gold. Each of these patents primarily is concerned with themanufacture of fixed bed catalyst useful for the manufacture of vinylacetate.

[0005] A new approach to produce vinyl acetate monomer is to use afluid-bed process in which gaseous reactants are contacted continuouslywith small supported catalyst particles under fluidized bed conditions.Expected benefits of a fluidized bed VAM process include a simpler fluidbed reactor design than a multi-tubular fixed bed reactor and increasedcatalyst life due to decreased hot spots which are typical of a fixedbed reactor. Further, continuous addition of make-up catalyst maintainscatalyst performance and eliminates complete catalyst change-out andshut-downs. Higher production rates may be achieved because higheroxygen levels safely may be fed into a fluid-bed reactor withoutproducing a flammable mixture. Recently-issued U.S. Pat. Nos. 5,591,688,5,665,667, and 5,710,318, assigned to the assignee of the presentinvention and incorporated by reference herein, are directed to theproduction of fluid bed vinyl acetate catalyst, or a fluid bed processfor the manufacture of vinyl acetate.

[0006] In any regard, conventional commercially acceptable VAM catalyst,whether used in fixed or fluid-bed reactor systems, uses gold incombination with the palladium metal species, such as described in U.S.Pat. No. 5,859,287 and European Published Application EP 0 723 810,incorporated by reference herein. It is believed that gold forms analloy with the palladium and inhibits agglomeration or sintering ofpalladium particles during the life of the catalyst under processconditions. Although other metals have been suggested as substitutes forgold in catalyst systems, gold has been found to be required forcommercially practicable catalyst in terms of activity and selectivity.However, gold is an expensive component in the catalyst preparation.Therefore, there is a need for a commercially useful catalyst that doesnot require, or minimizes, the presence of gold.

[0007] Further, there is a continuing need for VAM catalysts, especiallyfluid bed catalysts, which have more advantageous activity/selectivitycharacteristics and which are more resistant to attrition. As describedin this specification, the catalyst and catalyst support of thisinvention show commercially significant activity/selectivity propertieswithout a necessity of gold as a catalyst component. Further, catalystparticles of this invention typically show improved attrition resistanceunder normal fluid bed conditions.

SUMMARY OF THE INVENTION

[0008] A catalytically active material useful to produce vinyl acetatemonomer from ethylene, acetic acid, and an oxygen-containing gas underfluid bed conditions comprises a porous microspheroidal supportcontaining catalytically active palladium crystallites finely dispersedwithin the support. The catalyst material does not require incorporationof gold to maintain activity and selectivity.

[0009] A process to produce a vinyl acetate fluid bed catalyst in whichcatalytically active small palladium crystallites are finely dispersedwithin the support comprises dispersing selected metal species withinthe support which have an affinity to palladium to form very finecrystallites of palladium. The affinity metal species may be dispersedby impregnation onto a preformed microspheroidal support or may beintimately incorporated within the support before impregnation with asoluble palladium species.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0010] The vinyl acetate catalysts of this invention, suitable for usein a fluid bed reactor system, contain catalytically active palladiumcrystallites finely dispersed within microspheroidal supports. It isbelieved that small, finely dispersed crystallites maintain catalyticactivity and selectivity without a necessity of incorporation of gold inthe catalytic material.

[0011] In one aspect of this invention, catalytically-active palladiumcrystallites are incorporated within a support particle, suitable foruse in a fluid bed reactor system, such that the palladium crystallitesare well dispersed in an interior region of the particle. In preferablecatalysts, palladium crystallites are contained in an interior region ofthe catalyst particle and not concentrated at the surface. Althoughthere may be a gradation of palladium crystallite concentration frombelow the surface to the particle center, the palladium crystallitesappear finely dispersed in TEM photomicrographs, i.e., the palladiumcrystallites are substantially evenly distributed within the regionwithout prominent agglomerations. In comparison to similarly producedparticles using palladium and gold, which show significant numbers ofagglomerated Pd/Au crystallites, a preferable catalyst particle of thisinvention shows few, if any, agglomerated palladium crystallites.

[0012] Typically, palladium crystallites in catalyst particles of thisinvention are no more than about 20 nanometers (nm) in mean diameter. Inpreferred catalysts of this invention, reduced metal crystallites in thecatalyst particle including palladium crystallites are less than about15 nm and more preferably less than about 10 nm. Typical crystallitesare between about 5 to about 15 nm.

[0013] In order to form the fine palladium crystallites within amicrospheroidal support according to this invention, a metal speciesthat binds or has an affinity to palladium must be well dispersed within the support particle. These affinity metal species includelanthanides such as lanthanum and cerium and Group 3 and Group 4 (IUPACPeriodic Series system) metals such as titanium and zirconium. Unlikeuse of gold that forms a gold/palladium alloy, these affinity metals,when well dispersed into the support particles, do not formagglomerations of palladium crystallites. Thus, without agglomerationthere should be a greater palladium crystallite surface area availablefor catalytic sites.

[0014] According to this invention, one method to prepare thecatalytically active supported materials comprises contacting solutionsof palladium and the added affinity metal species with a preformedparticulate support. All of the metal species should be completelydissolved in a suitable solvent medium, preferably water, atsufficiently low temperature that agglomerations of metal do notaccumulate on the support particle during preparation. Preferably,impregnation with a soluble metal species is conducted at ambienttemperature. Thus, the solvent medium and the metal species are chosento achieve complete solubility, preferably at ambient temperatures, suchas 10 to 40° C. and usually 20 to 30° C. As described below, theimpregnated support is reduced to form metal crystallites within thesupport particle that are believed to be the catalytically active sites.Preferably, the reduction step follows the impregnation, although theremay be intermediate drying of the catalyst particles to facilitatehandling of the materials. There is no need to “fix” the soluble metalsalts onto the support with alkaline species as is necessary to form“shell” fixed bed catalysts before reduction.

[0015] In another method to prepare catalytically active supportedmaterials, the affinity metal is intimately distributed throughout thesupport material, in contrast to a mere physical mixture of twomaterials or an affinity metal species impregnated into the pores of thesupport and then calcined. For example, in the preparation of a poroussilica support useful in this invention, an oxide of an affinity metalsuch as cerium oxide, titanium oxide, or zirconium oxide may beincorporated in the silica sol during support preparation. In apreferred method, an oxide of the affinity metal replaces a portion ofsilica particles incorporated into the silica sol prior to spray dryingwhich forms the preformed support particles used in this invention. Inanother preferred embodiment, a sol of an affinity metal oxide may beused in preparation of a preformed catalyst support. Mixtures ofaffinity metal oxides may be incorporated into the preformed supportsused in this invention. In the preformed catalyst supports of thisinvention containing an intimate distribution of an affinity metaloxide, the affinity metal oxide is part of the porous support structureas an oxide.

[0016] In a further embodiment, solutions of affinity metal species maybe impregnated onto preformed support particles containing affinitymetal oxides in intimate association with the support structure.

[0017] The catalyst useful in this invention is supported on amicrospheroidal particulate material suitable for use in a fluid bedprocess. As is well known in the fluid bed art, these particles must besmall enough to be maintained in a fluid bed state under reactionconditions while keeping sufficient attrition resistance such thatexcessive amounts of catalyst need not be replenished during theprocess. Further, although typical particle sizes (as measured by meanparticle diameters) should not be so large as to be difficult to keep ina fluid bed state, there should not be an excess amount of very smallparticles (fines) which are difficult to remove from the system and mayplug gas recycle lines. Thus, typically suitable fluid bed catalystparticles have a distribution of larger to smaller particle sizes withinthe particle size limits.

[0018] In the process of this invention, typically, at least 50% of theparticles are less than about 105 microns, preferably at least 75% ofthe particles are less than 105 microns and more preferably at least 85%are less than 105 microns. In a typical catalyst useful in thisinvention, there may be less than 1 to 5% of particles more than 105microns. Further, typically, less than 50% are less than 44 microns andpreferably less than 35% are less than 44 microns. A typical catalystmay contain about 25 to 30% of the particles less than 44 microns. Atypical catalyst useful in this invention has at least 50% of theparticles with mean diameters between 44 and 88 microns. Persons skilledin the art will recognize that particles sizes of 44, 88, and 105microns are arbitrary measures in that they are based on standard sievesizes. Particle sizes and particle size distributions may be measured byan automated laser device such as a Microtrac 100.

[0019] Microspheroidal particles useful in this invention aresufficiently porous to permit gaseous reactants to diffuse into theparticle and contact catalytic sites incorporated within the particle.Thus, the pore volume should be high enough to permit gaseous diffusion.However, a particle with an exceedingly high pore volume typically willnot have sufficient attrition resistance or will not have sufficientsurface area for catalytic activity. A typically sufficientmicrospheroidal particle has a pore volume (measured by mercuryporosimetry) between about 0.2 and 0.7 cc/gram. A preferable particlehas a pore volume between about 0.3 and 0.65 cc/g and more preferablybetween about 0.4 and 0.55 cc/g.

[0020] Surface areas (measured by BET) for particles with mean diametersand pore volumes useful in this invention typically are above about 50m²/g and may range up to about 200 m²/g. A typical measured surface areais about 60 to about 125 m²/g.

[0021] Although silica-based supports are the most preferred in thisinvention, other oxides may be used as long as a particle of appropriatesize and with sufficient pore volume is produced on which may be placedthe required catalytic materials. Possible oxides include alumina,silica-alumina, titania, zirconia and mixtures thereof.

[0022] Typically useful supports, especially silica supports aredescribed in U.S. Pat. No. 5,591,688, incorporated by reference herein.In these supports a microspheroidal particle is produced by spray dryinga mixture of a silica sol with silica particles followed by drying andcalcining. In the preparation, at least 10 wt. %, preferably at least 50wt. %, of a silica sol is mixed with particulate silica. A usefulparticulate silica is a fumed silica such as Aerosil® (De Gussa ChemicalCompany).

[0023] A typical silica particulate material has a high surface area(about 200 m²/g) with essentially no micropores, and, typically, areaggregates (with mean diameters of several hundred nm) of individualparticles with average diameters of about 10 nm (above 7 nm).Preferably, the silica is sodium free. Sufficient particulate silica isadded to the mixture to obtain a desired pore volume in the resultingsupport particle. The amount of particulate silica may range up to 90wt. % and typically ranges up to 10 to 50 wt. % of the silica in themixture. Typically, the silica sol/particulate silica mixture is spraydried at an elevated temperature such as between 115° to 280° C.,preferably 1300 to 240° C., followed by calcining at temperaturetypically ranging from between 550° to 700° and, preferably 630° to 660°C.

[0024] As part of this invention, a portion or all of the particulatesilica may be replaced by an affinity metal species such as an oxide ofcerium, titanium, zirconium, or lanthanum. Typically, 0.5 to 20 wt. % ormore, preferably 1 to 5 wt. % of the particulate silica is substitutedwith these oxides.

[0025] Alternatively, a sol may be produced from an oxide other thansilica or in combination with silica. In this embodiment, a particulateoxide is added to a sol, such as a sol of ceria, titania, zirconia, asdescribed above for silica materials, and the resulting mixture spraydried to produce a preformed catalyst support particle. The particulatematerial may be silica, an oxide of an affinity metal, or a combinationthereof. Other compatible metal oxides may be present so long as therealso is a sufficient distribution of affinity metal in the particle toform the catalyst of the invention. The resulting particle should bemicrospheroidal and porous as described and should contain the affinitymetal oxide well distributed throughout the catalyst particle, such thatincorporation of palladium and subsequent reduction will form adistribution of palladium crystallites according to this invention.

[0026] Although catalysts of this invention typically may not requirethe presence of gold for activity and selectivity, gold may be added asan optional component, especially to maintain long-term stability orintegrity. Gold may be a useful component in a catalyst particle inwhich an affinity metal (e.g., Ce) is incorporated into the preformedsupport during preparation. However, the amount of gold typically willbe less than used in conventional catalysts and could be present inamounts up to 5 wt. %, preferably up to 3 wt. %, and many times lessthan 1 wt. %, in the catalyst material.

[0027] An advantageous silica sol useful in this invention containssilica particles in the sol typically more than 20 nanometers in meandiameter and may be up to about 100 nanometers or more. Preferable solscontain silica particles of about 40 to 80 nanometers. Nalco silica sol1060 particularly is advantageous because of the relatively large meansilica particle sizes of 60 nm pack less efficiently than smaller solparticles such as Nalco 2327 at about 20 nm. The larger particle sizesol yields a final support with higher mesopore volume and lessmicropore volume.

[0028] A suitable catalyst also contains an alkali metal (mostpreferably potassium) salt as a promoter up to about 10 wt. %,preferably up to 5 to 8 wt. %, and more preferably up to about 4 wt. %(calculated as alkali metal). Typically, at least 0.1 wt. % of alkalimetal is present in the catalyst and more preferably at least 1 wt. %. Atypical catalyst composition contains 0.5 to 2 wt. % palladium and 1 to3 wt. % potassium. The preferred salt is acetate. Usually, the alkalimetal salt is added as a solution, using the incipient wetness techniqueto control the amount of alkali metal salt placed on the catalystparticle, after impregnation of the palladium species and subsequentreduction. In an alternate embodiment, the alkali metal may be added tothe first impregnation solution.

[0029] A catalyst useful in this invention typically contains at leastabout 0.1 wt. %, preferably at least 0.2 wt. % palladium to about 5 wt.% and preferably up to 4 wt. % palladium. As indicated above, palladiumis incorporated into the support material preferably by incipientwetness to control the amount of palladium on the support.

[0030] The amount of affinity metal used typically is commensurate(although not necessarily equivalent) with the amount of palladium to beincorporated into the catalyst. A catalyst may contain at least about0.1 wt. %, preferably at least 0.2 wt. % affinity metal to about 10 wt.% or more and preferably up to 5 wt. % of the metal.

[0031] In preparation of a catalyst of this invention, advantageously,the impregnated metal species incorporated within the support, such aspalladium and cerium species, are reduced by contact with a suitablereducing agent. This reduction will transform the impregnated palladiumspecies to catalytically active zero valence palladium (Pd(0))crystallites. Typical reducing agents known to the art include hydrogen,hydrides, alkanes, alkenes, hydrazine, and the like. Preferably,hydrazine (most preferably in an aqueous solution) is used to reduce themetal species. Reduction with aqueous hydrazine after impregnation ispreferable. Typically, an excess of reducing agent is used to completethe reaction.

[0032] Preferably, impregnated and reduced catalyst particles are washedwith a suitable solvents such as water to remove excess reducing agentas well as undesired anions such as halides. Washing may be performedseveral times with portions of wash liquid until the desired level ofcontaminants is reached. Typically, the washed particles are driedslowly before addition of a promoter such as potassium acetate.

[0033] A preferable method to prepare the catalyst of this inventioncomprises contacting solutions of palladium and of at least one affinitymetal with a preformed porous microspheroidal support. The metal speciesshould be completely dissolved in the solvent medium at sufficiently lowtemperatures such that agglomeration of metal species does notaccumulate in the support particle during preparation. Preferably, theimpregnation with soluble metal species is conducted at ambienttemperature. Thus, the solvent medium and the metal species are chosento achieve complete solubility, preferably, at ambient temperature.Typical useful metal salts contain halide and the typical solvent isdeionized or distilled water. Typical soluble salts useful in thisinvention include salts of tetrachloropalladic acid, such as sodium orpotassium tetrachloropalladate, palladium chloride or palladium chloridedihydrate, palladium selenate, palladium sulfate, tetrammininepalladium(III) chloride, and the like. Tetrachloropalladate is preferred.Similarly, other soluble metal salts of affinity metals, such aschlorides, bromides, iodides, nitrates may be used. Halide salts,preferably chloride salts, typically are used. Since acetate salts ofpalladium and affinity metals are sparingly soluble in water or aceticacid, these salts typically would not be used in this invention.

[0034] These soluble metal salts may be impregnated onto the supportparticle through known procedures. A preferable method to impregnatesalt solutions is an incipient wetness technique in which an amount ofsalt solution measured to fill the pores of the support without excesssolution is used. Thus, a desired level of palladium and other metalspecies may be placed onto the support by calculating the amount ofmetals and the volume of solution needed to fill the pores. Since,typically, the solution impregnated support is allowed to dry slowlywithout washing, all of the metals in the impregnation solution will beincorporated into the support.

[0035] In a typical procedure, the preformed microspheroidal support isimpregnated with a solution (or solutions) of the metal salts (palladiumand at least one affinity metal) using an incipient wetness technique.Compounds of the active metal, palladium, and affinity metal componentare dissolved in the appropriate ratios in a suitable solvent. Thesupport material then is added to a solution containing thecatalytically active metal (Pd) and affinity metal species and stirredto allow the active metal and promoter element to impregnate themicrospheroidal support material. The impregnated catalyst support isdried slowly at an elevated temperature, such as 40 to 80° C., typicallyovernight. Preferably, the impregnated metal species are reduced to formactive palladium crystallites, washed to remove halide and reducingagent, and dried. The dried material is added to a second solutioncontaining a promoter alkali metal salt, preferably potassium acetate.This second solution is heated to evaporate the solvent to obtain adried catalyst as described above. The final, dry catalyst may be usedfor the production of vinyl acetate from feed preferably containingethylene, acetic acid, and an oxygen-containing gas in a fluid bedreactor system.

[0036] It is preferable that the impregnated metal species salts (Pd andaffinity metals) are dissolved in a single portion of solvent. Theamount of solvent is such that the pore volume of the support iscompletely filled with the first solution. However, in some instances adesired affinity metal may not be soluble in the same solvent as theother metal compounds to be used. In this case, a solution containingsome of the metal components may be impregnated upon the support,followed by impregnating a second solution containing the remainingcomponents. Solvents that are useful include water and volatile organicsolvents such as carboxylic acids with four carbons or fewer, alcohols,ethers, esters, and aromatics. The preferred solvent is water. In afurther embodiment of the present invention, the affinity metals may beplaced onto the finished catalyst by incorporating the affinity metalsduring the manufacture of the microspheroidal support.

[0037] The catalysts of the present invention may be used in a fluid bedreactor for the reaction of ethylene and acetic acid with oxygen toproduce vinyl acetate under fluid bed reaction conditions. The reactiontemperature suitably is maintained at about 100° to 250° C., preferably130° to 190° C. The reaction pressure suitably is about 50 to 200 psig(3 to 14 barg), preferably 75 to 150 psig (5 to 10 barg). In a fluid bedreactor system, the catalyst particles are maintained in a fluidizedstate by sufficient gas flow through the system. This gas flowpreferably is maintained at a level close to the minimum rate requiredto maintain the fluidization. Excess flow rate may cause channeling ofthe gas through the reactor which decreases conversion efficiency.Additional alkali metal salt promoter may be added during process tomaintain activity.

[0038] The following Examples illustrate but do not limit the inventiondescribed and claimed herein.

EXAMPLES 1-9 AND COMPARATIVE RUN A

[0039] A series of Example and Comparative Run experiments was conductedto test catalyst materials of this invention. In these experiments, acompletely dissolved aqueous solution of sodium tetrachlorpalladate incombination with a completely dissolved aqueous solution of a selectedaffinity metal was impregnated onto a preformed microspheroidal support(either a Support 1 or Support 2 material described below) by anincipient wetness technique. In this technique a measured amount ofimpregnating solution was contacted at ambient temperature with thesupport in an amount determined only to fill the pores of the supportwithout excess liquid. The resulting impregnated solid was dried at 60°C. overnight. The dried solid which incorporated the metal species wasreduced by contact with an aqueous hydrazine solution (prepared as 3grams of hydrazine hydrate per 80 ml of water) to reduce the metalspecies and the resulting solution was filtered and washed multipletimes with deionized water to remove hydrazine and residual chloride asconfirmed by a silver nitrate test. The resulting solid was dried at 60°C. overnight and further impregnated by incipient wetness with anaqueous solution of potassium acetate in an amount to provide thedesired amount of potassium in the catalyst and dried at 60° C.overnight. About two grams of the resulting catalyst material wascombined with an inert diluent (Ce/K or Au/K on Support 1 which had beenshown to be inert under the reaction conditions used) to produce about30 cc of total solid. This total solid was charged to the microreactoras described below. Results are shown in Table 1.

[0040] Support Preparation

[0041] Two types of preformed microspheroidal supports were prepared andused in the examples of present invention: (1) support materialcomprising 100% silica and (2) support material comprising silica incombination with other known inert support materials such as alumina,ceria, titania and zirconia. Prior to use, the supports were sieved anda specific particle size distribution of the support was used in thecatalyst preparations:

[0042] 5% of the particles are less than 105 microns but greater than 88microns

[0043] 70% of the particles are less than 88 microns but greater than 44microns

[0044] 25% of the particles are less than 44 microns

[0045] Support 1

[0046] Support 1 was prepared by spray drying a mixture of Nalco (NalcoChemical Company) silica sol 1060 and DeGussa Aerosil® (DeGussa ChemicalCompany) 200 silica according to U.S. Pat. No. 5,591,688. In the driedsupport, 80% of the silica came from the sol and 20% of the silica camefrom the Aerosil®. The spray dried microspheres were calcined in air at640° C. for 4 hours.

[0047] Support 2

[0048] A series of supports was prepared by spray drying a mixture ofNalco (Nalco Chemical Company) silica sol 1060, Degussa (DegussaChemical Company) Aerosil® 200 silica, and an additional oxide such ascerium oxide, titanium dioxide, zirconium oxide, aluminum oxide, orsilica/aluminum oxide mixtures (such as Aerosil® MOX 170 or Aerosil® COK84). In the dried support, 80% of the silica came from the sol, 20% ofthe silica came from the Aerosil®, and 1 to 3% by weight of the Aerosil®was replaced by oxides cerium or titanium. The spray driedmicrospheroidal support containing cerium was calcined in air at 640° C.for 4 hours.

[0049] Reactor Testing

[0050] The prepared catalysts were tested in a bench scale fluid bedreactor with a maximum catalyst capacity of 40 cubic centimeters.Sufficient catalyst was used such that the oxygen conversion was limitedto 30% in order to directly compare catalyst activity. A total catalystloading of 30 cubic centimeters volume was obtained by mixing sufficientinert microspheroidal material described above with an active catalystprior to reactor testing. The reactor was equipped with two feed inletswith ethylene, acetic acid, oxygen, and some nitrogen entering thereactor through the lower inlet and nitrogen only fed through a centralinlet.

[0051] Reactor pressure was controlled via a back-pressure regulatorreactor temperature was maintained at 152° C. and all lines leading toand from the reactor were heat traced and maintained at 160±5° C.

[0052] The gaseous reactor effluent was analyzed on-line using a HewlettPackard Model 5890 gas chromatograph equipped with boththermoconducitivity (TCD) and flame ionization (FID) detectors. Oxygen,nitrogen, ethylene and carbon dioxide were separated on a 13X molecularsieve column parallel with 23% SP1700 on 80/100 Chromosorb PAW, andquantified with the TCD. Vinyl acetate and acetic acid were separated ona 4% DP-1701 capillary column and quantified with the FID.

[0053] Activity (grams of vinyl acetate product per kilogram of catalystper hour) and selectivity (moles of vinyl acetate product per mole ofethylene feed) were calculated from these data. TABLE 1 ImpregnatedAffinity Activity Selectivity Support affinity metal in g. of VAM/ VAMto Example Type - Au Pd K metal support Kg. of cat./ ethylene (Run)Diluent (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) hr. (%) A I-Au/K 0.651.60 2.80 — — 1617 94.6 1 I-Ce/K — 1.60 2.74 Ce (0.60) — 2174 94.8  2¹I-Ce/K — 0.81 2.65 Ce (1.16) — 2774 91.8 3 I-Au/K — 1.61 2.80 La (0.65)— 1772 93.9 4 II-Ce/K 0.56 1.38 2.51 — Ce (0.2) 1857 94.6 5 II-Ce/K —1.41 2.52 — Ce (0.2) 1846 98.9 6 II-Ce/K — 1.50 2.66 Ce (0.57) Ce (0.6)1752 92.1 7 II-Au/K 0.54 1.42 2.67 — Ti (0.4) 1858 91.9 8 II-Au/K — 1.442.66 — Ti (0.6) 2158 94.2 9 II-Ce/K — 1.41 2.70 Ce (0.65) Ti (0.6) 200794.7

[0054] The data illustrate that catalysts of this invention maintainactivity and selectivity without the necessary presence of gold.Further, catalysts of this invention incorporating cerium showed anincreased attrition resistance during fluid bed testing.

[0055] These examples and accompanying specification have illustratedthe invention in terms of specific embodiments. Those skilled in the artrecognize modifications and variations to be within the intended scopeof the following claims.

We claim:
 1. A method to prepare a catalyst material useful to producevinyl acetate in a fluid bed reactor system comprising: contacting apreformed porous microspheroidal support with solutions of a palladiumcompound and at least one affinity metal compound such that thepalladium and affinity metal are finely dispersed into themicrospheroidal support; reducing the palladium compound to Pd(0)crystallites; adding a promoter amount of an alkali metal salt to thesupport; and recovering the resulting catalyst material.
 2. The methodof claim 1 wherein the affinity metal comprises a Group 3 or 4 metal ora lanthanide.
 3. The method of claim 1 wherein the affinity metal iscerium or lanthanum.
 4. The method of claim 1 wherein the affinity metalis titanium or zirconium.
 5. The method of claim I wherein the promoteralkali metal salt is potassium acetate.
 6. The method of claim 1 whereinthe preformed microspheroidal support is contacted with solutions ofhalide-containing salts of palladium and affinity metal.
 7. The methodof claim 1 wherein the preformed porous microspheroidal support iscontacted with a solution of a palladium compound and a solution of atleast on affinity metal compound in separate steps.
 8. The method ofclaim 1 wherein an affinity metal oxide is intimately incorporatedwithin the preformed microspheroidal support prior to impregnation witha palladium compound.
 9. The method of claim 1 in which the preformedmicrospheroidal support comprises silica.
 10. The method of claim 1wherein the solutions of a palladium compound and affinity metalcompound are aqueous solutions.
 11. The method of claim 1 which thecatalyst material does not contain gold.
 12. The method of claim 1wherein the palladium crystallites are less than about 10 nanometers inmean diameters.
 13. A catalyst material prepared according to claim 1.14. A catalyst material prepared according to claim
 3. 15. A catalystmaterial prepared according to claim
 8. 16. A catalyst material preparedaccording to claim
 11. 17. A method to prepare a catalyst materialuseful to produce vinyl acetate in a fluid bed reactor systemcomprising: preforming porous microspheroidal support particles in whichan affinity metal species is intimately dispersed within themicrospheroidal support particles; contacting the pre-formed porousmicrospheroidal support with a solution of a palladium compound suchthat the palladium metal is finely dispersed into the microspheroidalsupport; reducing the palladium compound to Pd(0) crystallites; adding apromoter amount of an alkali metal salt; and recovering the resultingcatalyst material.
 18. The method of claim 17 wherein the affinity metalincorporated into the preformed support comprises a Group 3 or 4 metalor a lanthanide.
 19. The method of claim 18 wherein the affinity metalis cerium, lanthanum, titanium, or zirconium.
 20. The method of claim 17wherein the solution of a palladium compound is an aqueous solution. 21.The method of claim 17 in which porous microspheroidal support particlesare preformed by adding an oxide of an affinity metal with silicaparticles to a silica sol and spray drying to form porousmicrospheroidal particles.
 22. The method of claim 17 in which porousmicrospheroidal support particles are preformed by adding silicaparticles to a sol of an oxide of an affinity metal and spray drying toform porous microspheroidal particles.
 23. The method of claim 22 inwhich an oxide of an affinity metal with silica particles are added tothe sol.
 24. The method of claim 21 wherein affinity metal oxide is anoxide of cerium or titanium or a mixture thereof.
 25. The method ofclaim 18 wherein the promoter alkali metal salt is potassium acetate.26. The method of claim 17 in which solutions of at least one affinitymetal compound are impregnated onto the microspheroidal support.
 27. Themethod of claim 26 in which a solution of a cerium compound isimpregnated onto the microspheroidal support.
 28. The method of claim 17which the catalyst material does not contain gold.
 29. A catalystmaterial prepared according to claim
 17. 30. A catalyst materialprepared according to claim
 21. 31. A catalyst material preparedaccording to claim
 27. 32. A catalyst useful for production of vinylacetate in a fluid bed reactor system comprising catalytically activepalladium crystallites incorporated in a microspheroidal supportstructure with an alkali metal salt promoter in which the palladiumcrystallites are finely dispersed throughout the microspheroidalstructure.
 33. The catalyst of claim 32 which does not contain gold. 34.The catalyst of claim 32 wherein the mean diameters of palladiumcrystallites are less than about 10 nanometers.
 35. The catalyst ofclaim 32 which contains an effective amount of at least one affinitycomponent comprising a Group 3 or 4 metal or a lanthanide.
 36. Thecatalyst of claim 35 in which the affinity component is lanthanum orcerium.
 37. The catalyst of claim 35 in which the affinity component istitanium or zirconium.
 38. The catalyst of claim 35 in which theaffinity component is impregnated onto the support.
 39. The catalyst ofclaim 35 in which the affinity component is incorporated into thesupport during support preparation.
 40. The catalyst of claim 32 whereinthe microspheroidal support is a silica or silica/alumina.
 41. Thecatalyst of claim 32 wherein the microspheroidal support is a poroussilica having a distribution of particle sizes such that at least 50% ofthe particles have mean diameters less than 105 microns and at least 50%of the particles have mean diameters between 44 and 88 microns.
 42. Thecatalyst of claim 32 wherein the microspheroidal support has a porevolume between about 0.2 and 0.7 cc/gram.
 43. The catalyst of claim 32wherein the microspheroidal support has a surface area above about 50m²/gram.
 44. The catalyst of claim 33 in wherein the porous preformedmicrospheroidal support is a silica with a pore volume between about 0.3to about 0.65 cc/gram which contains cerium as an affinity component.45. A catalyst of claim 44 which contains about 1 to 5 wt. % potassium.46. The catalyst of claim 45 in which at least a portion of the ceriumis incorporated into the preformed microspheroidal support.
 47. Thecatalyst of claim 46 wherein a portion of the cerium is impregnated ontothe preformed microspheroidal support.
 48. The catalyst of claim 45wherein the porous preformed microspheroidal support has a distributionof particle sizes such that at least 50% of the particles have meandiameters less than 105 microns, at least 50% of the particles have meandiameters between 44 and 88 microns, and no more than 5% or theparticles have a mean diameter over 105 microns and the palladiumcrystallites are less than 10 nm in mean diameter and comprise 0.2 to 4wt. % of the catalyst.
 49. The catalyst of claim 48 wherein at least aportion of the cerium is incorporated into the preformed microspheroidalsupport and a portion of the cerium is impregnated onto the preformedmicrospheroidal support.
 50. A method to produce vinyl acetatecomprising contacting ethylene, acetic acid, and an oxygen-containinggas with a catalyst of claim 1 in a fluid bed reactor under fluid bedreaction conditions.
 51. A method to produce vinyl acetate comprisingcontacting ethylene, acetic acid, and an oxygen-containing gas with acatalyst of claim 17 in a fluid bed reactor under fluid bed reactionconditions.
 52. A method to produce vinyl acetate comprising contactingethylene, acetic acid, and an oxygen-containing gas with a catalyst ofclaim 32 in a fluid bed reactor under fluid bed reaction conditions.